Recently, projectors that use a digital micro-mirror device (hereinafter DMD) array instead of an LCD display have come to be known. A DMD array is made by forming highly-reflective, rectangular, micro-mirror elements on a silicon chip using CMOS semiconductor technology. The inclination of the surface of each mirror element can be changed about 10 degrees or more according to a video signal input to the chip. A projector using a DMD array can modulate a light beam with image information by controlling the direction of light reflected from the mirror surfaces in the DMD array, thereby concentrating only desired reflected light onto a screen so as to project a desired image.
A DMD array can provide several millions (or more) of mirrors in rows and columns on a substrate, with each mirror being controllable independently with digital signals. Thus, each mirror can modulate the light in one pixel of an image. By this means, a projector using a DMD array can provide high quality images having a significantly larger number of pixels than provided by conventional LCD projectors. Also, because the illuminating light does not need to be polarized, as required in LCD displays, DMD arrays have a relatively small loss of light from the light source to the display screen. In addition, with DMD arrays, accurate gradation of the intensity of light in each pixel is made possible. Thus, there is increasing demand for this type of display technology, especially for portable displays which are convenient to carry.
DMD display technology has created market demand for a projection lens that is compatible with such display technology. Thus, there is market demand for a projection lens that is compact, provides high-performance imaging as a result of the various aberrations being well-corrected, and provides a wide angle of view.
U.S. Pat. No. 5,113,288 to Ohshita discloses a photographic lens having a wide angle of view in which there are four lens elements. There is no disclosure in this patent that the lens could be used as a projection lens. Furthermore, whereas for ease of manufacture of the second lens element L2 it might be obvious to form this lens element instead of two lens elements that are then joined. However, the two lens elements one of ordinary skill in the art would find obvious to substitute for the lens element L2 would undoubtedly have positive and negative refractive power, respectively, in order from the object side.
This invention relates to a projection lens used in projectors, especially to a projection lens for use with a projector that employs a DMD array. The object of the invention is to provide a projection lens having a pupil surface on the enlarging side (i.e, in the direction of the display screen) of the lens, to thereby enable the projection lens to achieve high performance imaging with a wide angle of view.